1. Field of Application
The present invention relates to a motor control apparatus for a motor having a rotor shaft that is coupled through a rotation transmission system such as a speed reduction mechanism to an output shaft that drives a control object.
2. Description of Prior Art
In recent years, there has been an increasing trend in the field of automobile technology towards replacing mechanical systems by systems which are electrically driven, i.e., by electric motors. This is done for reasons such as reduction of the amount of space required, ease of assembly, ease of system control, etc. This is exemplified in Japanese Patent Laid-open No. 2002-323127 (pages 3˜4, etc.), whereby an automatic transmission apparatus (referred to in the following simply as automatic transmission) of a vehicle is actuated to establish respective shift positions such as the drive (D) shift position, etc., through being driven by an electric motor (hereinafter referred to simply as a motor). The motor shaft is coupled through a speed reduction mechanism to an output shaft, which drives a shift position switching mechanism that directly acts on the automatic transmission to effect changeover from one shift position to another. The motor is provided with an encoder for detecting the rotation angle of the motor (i.e., of the motor shaft). When changeover of the shift position is to be performed, the motor shaft is rotated to a target value of rotation angle (expressed as a target count value of output pulses produced by the encoder) that corresponds to the target shift position (i.e., the shift position to which the change is to be effected). This is described in the above prior art patent, in pages 3˜4 thereof.
However when an amount of change in rotation angle of a motor shaft is converted to a corresponding amount of change in rotation angle of an output shaft, by being transferred through a rotation transmission system such as a speed reduction mechanism, there will be a certain degree of backlash in the components of the rotation transmission system, which typically include a gear train. As a result, even if the rotation angle of the motor shaft is controlled precisely, based on detecting that rotation angle by an appropriate sensor, there will be some amount of error in the rotation angle to which the output shaft becomes set, with the amount of error depending upon the degree of backlash. Hence, when an apparatus such as a shift position switching mechanism of an automatic transmission is actuated by the output shaft, it becomes difficult to achieve accurate control.
The assignees of the present invention have previously proposed, in Japanese Patent Laid-open No. 2002-177739, an arrangement whereby a motor (coupled through a rotation transmission system to an output shaft) is rotated until a member attached to the output shaft comes into contact with an immovable portion of a shift position switching mechanism, at a location corresponding to a limit position of a range of motion of the shift position switching mechanism. The amount of backlash in the rotation transmission system can thereby be “learned”, so that compensation can then be applied to a target value of rotation angle of the motor shaft, in accordance with the amount of backlash.
However in that case, even if the amount of backlash in the rotation transmission system is accurately learned, when the motor is subsequently driven to the target value of rotation angle after compensation has been applied for the backlash, an error may arise in the relationship between the respective rotation angles of the motor shaft and the output shaft, for amounts of rotation that are within the range of the backlash amount. This can result in errors of control. In addition, due to the fact that mechanical parts of the system are moved into mutual contact, in the aforementioned learning operation to determine the amount of backlash, loads are repetitively applied to parts of the mechanism, which can result in malfunction occurring. Moreover, due to the fact that the learning operation must be executed immediately before a shift position changeover of the automatic transmission can be performed, a delay will occur in executing the changeover operation. Hence, there are various disadvantages with such a method.